Thermostable blunt-end ligase and methods of use

ABSTRACT

Fusion proteins having thermostable blunt-end ligase activity are provided. Blunt-end ligases are useful for DNA amplification, sequencing, production of recombinant DNA and recombinant fusion proteins, and other purposes. These thermostable blunt-end DNA ligases are useful in ligation schemes which include, e.g., an incubation at about 60-65° C. or higher, or as high as about 94° C., or at other temperatures. The ligases disclosed herein may enable high temperature blunt-end ligation without need for molecular crowding agents, and so may be useful for many nucleic acid ligation-amplification schemes, e.g., ones which operate at a uniform temperature (e.g., at about 60° C. or higher), including ones which require temperature cycling, e.g., from about 94° C. to about 60° C. (or higher) for one, two, three, or more cycles. The thermostable blunt-end DNA ligases disclosed herein enable higher specificity target amplification, for example, by permitting temperature denaturation of double-stranded DNA templates as well as specific primer binding.

SEQUENCE LISTING

The instant application contains a Sequence Listing which has been submitted electronically in ASCII format and is hereby incorporated by reference in its entirety. Said ASCII copy, created on May 22, 2014, is named 2028.201_SL.txt and is 35,411 bytes in size.

BACKGROUND

A variety of methods for the amplification of nucleic acids are known. For example, polymerase chain reaction (“PCR”) (see, e.g. U.S. Pat. No. 4,683,202) is a popular method for the amplification of nucleic acids. To successfully perform a PCR reaction, the reaction must be performed at multiple different temperatures. This requires hardware or other mechanisms for repeatedly changing the temperature of the PCR reaction. Another method for amplification of nucleic acids is referred to as loop-mediated isothermal amplification (“LAMP”) (see, e.g. U.S. Pat. No. 6,410,278). LAMP reactions may be performed isothermally, but typically involve the use of four different primers which recognize a total of six distinct sequences on the target nucleic acid.

Some amplification methods utilize DNA ligation. DNA ligation is a common molecular biology method for joining multiple DNA fragments. Ligases can seal single-strand nicks in duplex DNA, join two pieces with complementary “sticky” ends, and in some cases join two pieces with blunt, non-sticky ends. These methods find use in molecular cloning, nucleic acid diagnostics/detection, nucleic acid amplification, and related activities. Ligases and ligation methods are described, for example, in U.S. Patent Application Publication No. 20120214208; U.S. Pat. No. 7,927,853; de Lumley et al., J. Mol. Biol. (2004) 339, 1059-1075; and Rolland et al., FEMS Microbiol. Lett. (2004) 236, 267-273.

Ligation of two strands of “sticky-ended” DNA is possible, where the substrates to be joined are already pre-positioned in the case of single-strand nick sealing, and the affinity of sticky ends for each other helps drive sticky-end ligation. However, since DNA ligation depends upon the juxtaposition of the two substrates to be joined, DNA ligation is more difficult in situations where the juxtaposition of the two substrates is more difficult, or the strands are likely to become mis-aligned or to separate readily. For this reason, blunt-end and high temperature ligations are two types of ligation that are particularly difficult. Blunt-end ligation, however, depends upon random interactions of the substrates. Two adjustments in blunt-end ligation are commonly made to drive the substrate interactions: low temperature to slow down molecular motion so that random interactions last longer and thus give the ligase a better chance to join the fragments; and molecular crowding reagents such as polyethylene glycol to increase the local concentrations of substrates. Likewise, high temperature ligations are challenging because the interactions of DNA substrates are briefer. The least efficient case is thus a high temperature blunt-end ligation in which molecular crowding reagents cannot be used.

A thermostable ligase, T4 DNA ligase, has been described that can perform blunt-end ligations; however, it is inactivated at temperature above approximately 45° C., so that the range of temperature at which T4 DNA ligase is stable is relatively small. A few other examples of ligases that can be induced to join blunt-ended fragments are known, but these ligases appear to do so only in the presence of high concentrations of molecular crowding agents such as 50% polyethylene glycol (PEG). However, the utility of ligases that require such molecular crowding agents is unclear, since 50% PEG which DNA polymerization (which is required for DNA amplification).

Accordingly, in order to facilitate the generation of amplified nucleic acids for the many and growing number of applications which use amplified nucleic acids, new methods and reagents for the amplification of nucleic acids are desired.

INCORPORATION BY REFERENCE

All publications, patents, and patent applications mentioned in this specification are herein incorporated by reference to the same extent as if each individual publication, patent, or patent application was specifically and individually indicated to be incorporated by reference.

SUMMARY

Fusion proteins having thermostable blunt-end ligase activity are provided. Such thermostable blunt-end ligases are useful for DNA amplification, sequencing, production of recombinant DNA (e.g., as a result of such blunt-end fusions) and recombinant fusion proteins (e.g., proteins encoded by recombinant DNA produced as a result of such blunt-end fusions), and for other purposes. Many molecular biology schemes involve ligases; such schemes would benefit from the ability to perform such schemes at elevated temperatures. Thermostable blunt-end DNA ligases disclosed herein are suitable for use in ligation schemes which would benefit from at least one elevated temperature incubation, such as an incubation at about 60-65° C. or higher, including incubation schemes at temperatures as high as about 94° C. A thermostable blunt-end ligase as disclosed herein would be useful for such schemes.

In addition, thermostable blunt-end ligases as disclosed herein may enable high temperature blunt-end ligation without the need for molecular crowding agents. Accordingly, thermostable blunt-end ligase as disclosed herein may be useful for many nucleic acid ligation-amplification schemes, including nucleic acid ligation-amplification schemes which operate at a uniform temperature (e.g., at about 60° C. or higher), and including nucleic acid ligation-amplification schemes which require temperature cycling such as cycling from, e.g., about 94° C. to about 60° C. (or higher) for one, two, three, or more cycles. Nucleic acid ligation-amplification schemes which may benefit from the use of thermostable blunt-end DNA ligases disclosed herein may operate at other temperatures as well, e.g., depending on the temperature resistance characteristics of the novel fusion proteins disclosed herein and the requirements of the difference nucleic acid ligation-amplification schemes. Thermostable blunt-end DNA ligases disclosed herein provide the ability to use high temperatures in nucleic acid ligation-amplification schemes and thereby enable higher specificity target amplification, for example, by permitting temperature denaturation of double-stranded DNA templates as well as specific primer binding.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 illustrates the results of a blunt-ended ligation reaction performed at 75° C., and demonstrates that fusion proteins as disclosed herein provide improved blunt-end ligation activity at high temperature.

DETAILED DESCRIPTION

Thermostable, blunt-end ligases are provided herein. As disclosed herein, a thermostable, blunt-end ligase may be prepared by the fusion of a DNA-binding protein to a thermostable ligase. Such ligases produced by these fusions have features and capabilities not provided by their parent compounds alone; the different portions of these fusion proteins provide activities which, when combined, provide new capabilities and unexpectedly improved activity. The DNA-binding protein portion of such fusion proteins is effective to increase the affinity of the ligase to DNA substrates, resulting in enhanced ligation in the challenging conditions of high temperature, blunt-end ligation. The DNA ligase portion surprisingly retains its ability to ligate DNA when combined with a foreign protein (the DNA-binding protein portion). Together, the combined portions unite in novel fusion proteins that are able to ligate blunt-ended DNA substrates even at high temperatures, providing increased ligation activity at high temperatures unavailable by the use of the original, unmodified ligases.

Methods, reagents, devices, systems, and articles of manufacture useful for the practice of the methods, and for the use of reagents, devices, systems, and articles of manufacture disclosed herein, are described, for example, in U.S. Patent Application Ser. No. 61/800,606 and U.S. Patent Application Ser. No. 61/800,925, the entire disclosures of which are hereby incorporated by reference in their entireties.

Definitions

Before the present novel ligases and ligation methods are disclosed and described, it is to be understood that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting. It is also to be understood that the present disclosure provides explanatory and exemplary descriptions and examples, so that, unless otherwise indicated, the molecules, compositions, assays, methods, and kits disclosed herein are not limited to the specific embodiments described herein.

It must be noted that, as used in the specification and the appended claims, the singular forms “a,” “an” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a salt” refers to a single salt or mixtures of different salts, and the like.

As used in the description herein and throughout the claims that follow, the meaning of “in” includes “in” and “on” unless the context clearly dictates otherwise.

As used in the description herein and throughout the claims that follow, the meaning of “or” includes both the conjunctive and disjunctive unless the context expressly dictates otherwise. Thus, the term “or” includes “and/or” unless the context expressly dictates otherwise.

In this specification and in the claims that follow, reference will be made to a number of terms, which shall be defined to have the following meanings:

The term “moiety” as used herein refers to any particular composition of matter, e.g., a molecular fragment, an intact molecule, or a mixture of materials.

The term “nucleic acid” refers to nucleotides and nucleosides which make up, for example, deoxyribonucleic acid (DNA) macromolecules and ribonucleic acid (RNA) macromolecules. Nucleic acids may be identified by the base attached to the sugar (e.g., deoxyribose or ribose); as used herein, the following abbreviations for these bases are used to represent nucleic acids in sequence listings identifying and describing their structures (either upper-case or lower-case may be used).

TABLE 1A Base (in Nucleic Acid) Letter Code Adenine A Thymine T Guanine G Cytosine C Uracil U

As used herein, a “polynucleotide” refers to a polymeric chain containing two or more nucleotides. “Polynucleotides” includes primers, oligonucleotides, nucleic acid strands, etc. A polynucleotide may contain standard or non-standard nucleotides. Typically, a polynucleotide contains a 5′ phosphate at one terminus (“5′ terminus”) and a 3′ hydroxyl group at the other terminus (“3′ terminus) of the chain. The most 5′ nucleotide of a polynucleotide may be referred to herein as the “5′ terminal nucleotide” of the polynucleotide. The most 3′ nucleotide of a polynucleotide may be referred to herein as the “3′ terminal nucleotide” of the polynucleotide.

The term “downstream” as used herein in the context of a polynucleotide containing a 5′ terminal nucleotide and a 3′ terminal nucleotide refers to a position in the polynucleotide which is closer to the 3′ terminal nucleotide than a reference position in the polynucleotide. For example, in a primer having the sequence: 5′ ATAAGC 3′, the “G” is downstream from the “T” and all of the “A”s.

The term “upstream” as used herein in the context of a polynucleotide containing a 5′ terminal nucleotide and a 3′ terminal nucleotide, refers to a position in the polynucleotide which is closer to the 5′ terminal nucleotide than a reference position in the polynucleotide. For example, in a primer having the sequence: 5′ ATAAGC 3′, the “T” is upstream from the “G”, the “C”, and the two “A”s closest to the “G”.

As used herein, “nucleic acid” includes both DNA and RNA, including DNA and RNA containing non-standard nucleotides. A “nucleic acid” contains at least one polynucleotide (a “nucleic acid strand”). A “nucleic acid” may be single-stranded or double-stranded.

As used herein, a nucleic acid molecule which is described as containing the “sequence” of a template or other nucleic acid may also be considered to contain the template or other nucleic acid itself (e.g. a molecule which is described as containing the sequence of a template may also be described as containing the template), unless the context clearly dictates otherwise.

As used herein, a “target” nucleic acid or molecule refers to a nucleic acid of interest. A target nucleic acid/molecule may be of any type, including single-stranded or double stranded DNA or RNA (e.g. mRNA).

As used herein, “complementary” sequences refer to two nucleotide sequences which, when aligned anti-parallel to each other, contain multiple individual nucleotide bases which pair with each other. It is not necessary for every nucleotide base in two sequences to pair with each other for sequences to be considered “complementary”. Sequences may be considered complementary, for example, if at least 30%, 40%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 98%, 99%, or 100% of the nucleotide bases in two sequences pair with each other. In addition, sequences may still be considered “complementary” when the total lengths of the two sequences are significantly different from each other. For example, a primer of 15 nucleotides may be considered “complementary” to a longer polynucleotide containing hundreds of nucleotides if multiple individual nucleotide bases of the primer pair with nucleotide bases in the longer polynucleotide when the primer is aligned anti-parallel to a particular region of the longer polynucleotide.

As used herein, the term “isolated” as applied to proteins, nucleic acids, or other biomolecules refers to a molecule that has been purified or separated from a component of its naturally-occurring environment (e.g. a protein purified from a cell in which it was naturally produced). An “isolated” molecule may be in contact with other molecules (for example, as part of a reaction mixture). As used herein, “isolated” molecules also include recombinantly-produced proteins or nucleic acids which have an amino acid or nucleotide sequence which occurs naturally. “Isolated” nucleic acids include polypeptide-encoding nucleic acid molecules contained in cells that ordinarily express the polypeptide where, for example, the nucleic acid molecule is at a chromosomal location different from that of natural cells. In some embodiments, “isolated” polypeptides are purified to at least 50%, 60%, 70%, 80%, 90%, 95%, 98%, or 100% homogeneity as evidenced by SDS-PAGE of the polypeptides followed by Coomassie blue, silver, or other protein staining method.

The terms “polypeptide” and “protein” may be used interchangeably to refer to molecules comprised of amino acids linked by peptide bonds. Individual amino acids may be termed “residues” of a polypeptide or protein. The amino acid sequences of polypeptides disclosed herein may be identified by SEQ ID NO: presented as a string of letters, where the letters have the following meanings:

TABLE 1B Amino Acid 3-Letter Code 1-Letter Code Alanine Ala A Arginine Arg R Asparagine Asn N Aspartic acid Asp D Cysteine Cys C Glutamic acid Glu E Glutamine Gln Q Glycine Gly G Histidine His H Isoleucine Ile I Leucine Leu L Lysine Lys K Methionine Met M Phenylalanine Phe F Proline Pro P Serine Ser S Threonine Thr T Tryptophan Trp W Tyrosine Tyr Y Valine Val V

A composition may include a buffer. Buffers include, without limitation, phosphate, citrate, ammonium, acetate, carbonate, tris(hydroxymethyl)aminomethane (TRIS), 3-(N-morpholino) propanesulfonic acid (MOPS), 3-morpholino-2-hydroxypropanesulfonic acid (MOPSO), 2-(N-morpholino)ethanesulfonic acid (MES), N-(2-Acetamido)-iminodiacetic acid (ADA), piperazine-N,N′-bis(2-ethanesulfonic acid) (PIPES), N-(2-Acetamido)-2-aminoethanesulfonic acid (ACES), cholamine chloride, N,N-Bis(2-hydroxyethyl)-2-aminoethanesulfonic acid (BES), 2-[[1,3-dihydroxy-2-(hydroxymethyl)propan-2-yl]amino]ethanesulfonic acid (TES), 4-(2-hydroxycthyl)-1-piperazine ethanesulfonic acid (HEPES), acetamidoglycine, tricine (N-(2-Hydroxy-1,1-bis(hydroxymethyl)ethyl)glycine), glycinamide, and bicine (2-(Bis(2-hydroxyethyl)amino)acetic acid) buffers. Buffers include other organic acid buffers in addition to the phosphate, citrate, ammonium, acetate, and carbonate buffers explicitly mentioned herein.

An article of manufacture may comprise a container; and a composition contained within the container, wherein the composition comprises a thermostable ligase. An article of manufacture may comprise a container; and a composition contained within the container, wherein the composition comprises a blunt-end ligase. An article of manufacture may comprise a container; and a composition contained within the container, wherein the composition comprises a thermostable blunt-end ligase. The containers may be formed from a variety of materials such as glass or plastic, and may have a sterile access port (for example the container may be an intravenous solution bag or a vial having a stopper pierceable by a hypodermic injection needle). The article of manufacture may further comprise a label or package insert on or associated with the container indicating that the composition may be used to ligate blunt-ended DNA.

Embodiments of methods and compositions provided herein may be described with reference to FIG. 1.

The novel thermostable blunt-end DNA ligases disclosed herein are comprised of the fusion of a DNA-binding protein with a thermostable ligase. Such fusion proteins as disclosed herein may further comprise other components, such as, for example, peptide linkers, N-terminal or C-terminal additions, tag peptides, and other amino acid sequences, including D-amino acids and peptide mimetics. In addition, fusion protein as disclosed herein may further comprise other components, such as, for example, sugars (e.g., the fusion proteins may be glycosylated), polymers such as polyethylene glycol (e.g., the fusion proteins may be PEGylated), organic moieties other than amino acids, and other additions linked to the fusion proteins. The DNA-binding protein portion increases the affinity of the ligase to DNA substrates, resulting in enhanced ligation in the challenging conditions of high temperature, blunt-end ligation. Many combinations of binding proteins and ligases are possible. For example, the binding protein may be N-terminal or C-terminal relative to the ligase.

It will be understood that multiple combinations of DNA ligases and DBA-binding proteins may be joined to produce the fusion proteins disclosed herein. For example, the following 16 combinations have been constructed:

TABLE 2 Fusion Proteins Constructed

* Light gray refers to the DNA-binding protein. Descriptions of CTF and p50 are given below. * Dark gray refers to the ligase. T4 = T4 phage, Tth = Thermus thermophilus, Pfu = Pyrococcus furiosus, Pfu C23 = Pfu with a C-terminal 23-amino acid deletion.

Additional variations are possible, including: (1) different DNA-binding proteins, including thermostable proteins; (2) other DNA ligases, including T4 DNA ligase variants engineered to be thermostable; (3) multiple DNA-binding proteins per ligase, or multiple ligases per DNA-binding protein; (4) transient, non-covalent linkages between the ligase and DNA-binding protein, rather than a protein fusion, to better enable multiple ligation events per ligase molecule; (5) dimers or higher-order multimers of fusion proteins to increase the local concentration of ligase once DNA substrates are bound; (6) different degrees of affinity between the DNA substrate and the DNA-binding protein—for example, using the natural target sequence of p50, GGGAATTCCC (SEQ ID NO: 1), in the DNA target, to enable low picomolar affinity; (7) other nucleic acid modifying enzymes instead of DNA ligase to perform other molecular reactions.

An exemplary method for producing the fusion proteins disclosed herein may be as follows. Fusion proteins may be made by inducing E. coli to express DNA constructs made by standard recombinant DNA methods, followed by standard chromatographic protein purification methods. An affinity tag such as polyhistidine may be employed to assist in the protein purification. In the case of thermostable proteins, purification may be assisted by employing heat to denature most E. coli host proteins. The purified fusion protein may be used in the same way that a standard, non-heat-stable ligase would be used in the application/reaction of choice, for example in a scheme that depends upon ligation of DNA substrates to make a template for amplification.

The methods disclosed herein can be readily incorporated into and used in device for processing a sample, or a system for processing a sample, which may be an automated assay device, or may be an automated assay system. Such assay devices and assay systems may comprise devices and systems disclosed, for example, in U.S. Pat. No. 8,088,593; U.S. Pat. No. 8,380,541; U.S. patent application Ser. No. 13/769,798, filed Feb. 18, 2013; U.S. patent application Ser. No. 13/769,779, filed Feb. 18, 2013; U.S. patent application Ser. No. 13/244,947 filed Sep. 26, 2011; PCT/US2012/57155, filed Sep. 25, 2012; U.S. application Ser. No. 13/244,946, filed Sep. 26, 2011; U.S. patent application Ser. No. 13/244,949, filed Sep. 26, 2011; and U.S. Application Ser. No. 61/673,245, filed Sep. 26, 2011, the disclosures of which patents and patent applications are all hereby incorporated by reference in their entireties.

EXAMPLES

Examples of individual protein components suitable for use in providing fusion proteins as disclosed herein include (the amino acid sequences are provided using the one-letter code for amino acids):

TABLE 3  SEQ ID Protein Name Protein Sequence NO: p50 DNA-binding adgpylqileqpkqrgfrfryvcegpshgglpgasseknk 2 protein (fragment from ksypqvkicnyvgpakvivqlvtngknihlhahslvgkhc the human NF-kappa-B edgictvtagpkdmvvgfanlgilhvtkkkvfetlearmt protein, accession eacirgynpgllvhpdlaylqaegggdrqlgdrekelirq number NP_003989, aalqqtkemdlsvvrlmftaflpdstgsftrrlepvvsda amino acids 40-366) iydskapnasnlkivrmdrtagcvtggeeiyllcdkvqkd diqirfyeeeenggvwegfgdfsptdvhrgfaivfktpky kdinitkpasvfvqlrrksdletsepkpflyypeikdkee vqrkrqk CTF DNA-binding sptsymspslpaldwqlpshsgpyelrievqpkshhrahy 3 protein (a hybrid from etegsrgavkasagghpivqlhgylenepltlqlfigtad the murine NFATc1 drllrphafyqvhritgktvsttsheiilsntkvleipll protein, accession pennmraiidcagilklrnsdielrkgetdigrkntrvrl number NP_058071, vfrvhipqpngrtlslqasnlkivrmdrtagcvtggeeiy amino acids 390-506; llcdkvqkddiqirfyeeeenggvwegfgdfsptdvhrqf followed by an alanine aivfktpkykdinitkpasvfvqlrrksdletsepkpfly spacer; followed by a ypeikdkeevqrkrqk fragment from the human NF-kappa-B protein, accession number NP_003989, amino acids 249-366) T4 DNA ligase Milkilneiasigstkqkqaileknkdnellkrvyrltys 4 (accession number rglqyyikkwpkpgiatqsfgmltltdmldfieftlatrk NP_049813) ltgnaaieeltgyitdgkkddvevlrrvmmrdlecgasvs iankvwpglipeqpqmlassydekginknikfpafaqlka dgarcfaevrgdelddvrllsragneylgldllkeelikm taearqihpegvlidgelvyheqvkkepegldflfdaype nskakefaevaesrtasngiankslkgtisekeaqcmkfq vwdyvplveiyslpafrlkydvrfskleqmtsgydkvili enqvvnnldeakviykkyidqglegiilknidglwenars knlykfkevidvdlkivgiyphrkdptkaggfilesecgk ikvnagsglkdkagvksheldrtrimenqnyyigkilece cngwlksdgrtdyvklflpiairlredktkantfedvfgd fhevtgl Tth DNA ligase mtleearkrvnelrdliryhnyryyvladpeisdaeydrl 5 (Thermus Thermophilus lrelkeleerfpelkspdsptlqvgarpleatfrpvrhpt strain HB8, accession rmysldnafnldelkafeerieralgrkgpfaytvehkvd YP_144363.1) glsvnlyyeegvlvygatrgdgevgeevtqnlltiptipr rlkgvperlevrgevympieaflrlneeleergerifknp rnaaagslrqkdpritakrglratfyalglgleeveregv atqfallhwlkekgfpvehgyaravgaegveavyqdwlkk rralpfeadgvvvkldelalwrelgytaraprfaiaykfp aeeketrlldvvfqvgrtgrvtpvgilepvflegsevsrv tlhnesyieeldirigdwvlvhkaggvipevlrvlkerrt geerpirwpetcpecghrllkegkvhrcpnplcpakrfea irhfasrkamdiqglgeklierllekglvkdvadlyrlrk edlvglermgeksaqnllrqieeskkrglerllyalglpg vgevlarnlaarfgnmdrlleasleelleveevgeltara iletlkdpafrdlvrrlkeagvemeakekggealkgltfv itgelsrpreevkallrrlgakvtdsysrktsylvvgenp gsklekaralgvptlteeelyrlleartgkkaeelv Pfu DNA ligase mrylelaqlyqklekttmkliktrlvadflkkvpddhlef 6 (Pyrococcus furiosus ipylilgdvfpewderelgvgekllikavamatgidanei strain ST04, accession ensvkdtgdlgesialavkkrkqksffsqpltikrvyqtl YP_006355162) vkvaettgegsqekkmkylanlfmdaepieakyiartvlg tmrtgvaegllrdaialafhvkvelveraymltsdfgfva kvaklegneglakvqvgigkpikpmlaqqaanikeallem ggeaefeikydgarvqvhkdgdkiivysrrlenvtraipe ivealkqsvkpnkaivegelvaigedgrplpfqyvlrrfr rkhniqemmkkiplelnlfdvlyvdgesmidvkfidrrkk leeiiepngkikvaenlitkkveeaeafykkalemghegl makrldatyepgnrgkkwlkikptmenldlviigaewgeg rrahllgsfilgaydpetgeflevgkvgsgftdedlveft kmlkpliikeegkrvwiepkivievtyqeiqkspkyksgf alrfpryvalrddkgpedadtieriaglyelgermkgkv Pfu C23 DNA ligase mrylelaglyqklekttmkliktrlvadflkkvpddhlef 7 (C-terminal 23-amino ipylilgdvfpewderelgvgekllikavamatgidanei acid deletion of Pfu) ensvkdtgdlgesialavkkrkqksffsgpltikrvyqtl vkvaettgegsqekkmkylanlfmdaepieakyiartvlg tmrtgvaegllrdaialafhvkvelveraymltsdfgfva kvaklegneglakvqvqigkpikpmlaqqaanikeallem ggeaefeikydgarvqvhkdgdkiivysrrlenvtraipe ivealkqsvkpnkaivegelvaigedgrplpfqyvlrrfr rkhniqemmkkiplelnlfdvlyvdgesmidvkfidrrkk leeiiepngkikvaenlitkkveeaeafykkalemghegl makrldatyepgnrgkkwlkikptmenldlviigaewgeg rrahllgsfilgaydpetgeflevgkvgsgftdedlveft kmlkpliikeegkrvwiepkivievtyqeiqkspkyksgf alrfpryvalrddkgp

The following provides an example of a fusion protein providing thermostable blunt-end DNA ligase activity:

-   -   p50-Tth ligase fusion, Theranos construct EE0217, e.g. protein         prep RDP0078, consists of a His10-containing leader (“His10”         disclosed as SEQ ID NO: 11), p50 (shown in italics), a flexible         glycine-rich sequence, and the Tth DNA ligase (shown         underlined).

mghhhhhhhhhhssghiegrasadgpylqileqpkqrgfrfryvcegpshgglpgasseknkksypqvkicnyvgpakvivqlvtng knihlhahslvgkhcedgictvtagpkdmvvgfanlgilhvtkkkvfetlearmteacirgynpgllvhpdlaylqaegggdrqlgdrek elirqaalqqtkemdlsvvrlmftaflpdstgsftrrlepvvsdaiydskapnasnlkivrmdrtagcvtggeeiyllcdkvqkddiqirfye eeenggvwegfgdfsptdvhrqfaivfktpkykdinitkpasvfvqlrrksdletsepkpflyypeikdkeevqrkrqkgssgtsgggsgg gmtleearkrvnelrdliryhnyryyvladpeisdaeydrllrelkeleerfpelkspdsptlqvgarpleatfrpvrhptrmysldnafnld elkafeerieralgrkgpfaytvehkydglsvnlyyeegvlvvgatrgdgevgeevtqnlltiptiprrlkgvperlevrgevympieaflrl neeleergerifknprnaaagslrqkdpritakrglratfyalglgleeveregvatqfallhwlIkekgfpvehgyaravgaegveavyqd wlkkrralpfeadgvvvkldelalwrelgytaraprfaiaykfpaeeketrlldvvfqvgrtgrvtpvgilepvflegsevsrvtlhnesyiee ldirigdwvlvhkaggvipevlrvlkerrtgeerpirwpetcpecghrllkegkvhrcpnplcpakrfeairhfasrkamdiqglgeklierl lekglvkdvadlyrlrkedlyglermgeksaqnllrqieeskkrglerllyalglpgvgevlarnlaarfgnmdrlleasleelleveevgelta railetlkdpafrdlyrrlkeagvemeakekggealkgltfvitgelsrpreevkallrrlgakvtdsvsrktsylvvgenpgsklekaralgvp tlteeelyrlleartgkkaeelv

An example of a blunt-ended ligation reaction performed at 75° C. is provided below. The DNA substrate was a 49-bp duplex DNA made by annealing oligonucleotides EE0139 and EE0140 (sequences below). This blunt-ended duplex was capable of making concatamers upon multiple ligation events. The results of the reaction products were separated by size on an agarose gel, as shown below, demonstrate that: (1) Tth ligase alone (lanes 2-3), although it is known to be capable of sealing DNA nicks at 75° C., performed very little or no blunt ligation in these conditions; (2) T4 DNA ligase with an N-terminal p50 fusion (lanes 4-5) also performed very little blunt ligation in these conditions, although there was some evidence of more ligation than for Tth alone. This observation was believed to be surprising given the temperature sensitivity of T4 ligase alone; (3) Tth DNA ligase with an N-terminal p50 fusion (lanes 6-7) demonstrated a much higher level of blunt-end ligation at 75° C. Thus, the results shown in FIG. 1 below demonstrate that fusion proteins as disclosed herein provide improved blunt-end ligation activity at high temperature. 

1. A thermostable blunt-end DNA ligase comprising a fusion protein comprising a DNA ligase and a DNA binding protein, wherein said thermostable blunt-end DNA ligase is suitable for use in a blunt-ended DNA ligation reaction performed at about 60° C. or higher.
 2. The thermostable blunt-end DNA ligase of claim 1, comprising a T4 DNA ligase with an N-terminal p50 fusion.
 3. The thermostable blunt-end DNA ligase of claim 1, comprising the amino acid sequence: (SEQ ID NO: 8) mghhhhhhhhhhssghiegrasadgpylqileqpkqrgfrfryvcegpshgglpgasseknkksypqvkicnyvgpakvivqlvtngknihlh ahslvgkhcedgictvtagpkdmvvgfanlgilhvtkkkvfetlearmteacirgynpgllvhpdlaylqaegggdrqlgdrekelirqaalqqtke mdlsvvrlmftaflpdstgsftrrlepvvsdaiydskapnasnlkivrmdrtagcvtggeeiyllcdkvqkddiqirfyeeeenggvwegfgdfspt dvhrqfaivfktpkykdinitkpasvfvqlrrksdletsepkpflyypeikdkeevqrkrqkgssgtsgggsgggmtleearkrvnelrdliryhnyr yyvladpeisdaeydrllrelkeleerfpelkspdsptlqvgarpleatfrpvrhptrmysldnafnldelkafeerieralgrkgpfaytvehkvdgls vnlyyeegylvygatrgdgevgeevtqnlltiptiprrlkgyperlevrgevympieaflrlneeleergerifknprnaaagslrqkdpritakrglra tfyalglgleeveregvatqfallhwlkekgfpvehgyaravgaegveavyqdwlkkrralpfeadgvvvkldelalwrelgytaraprfaiaykf paeeketrlldvvfqvgrtgrvtpvgilepvflegsevsrvtlhnesyieeldirigdwvlvhkaggvipevlrvlkerrtgeerpirwpetcpecghrl lkegkvhrcpnplcpakrfeairhfasrkamdiqglgekliellekglvkdvadlyrlrkedlvglermgeksaqnllrqieeskkrglerllyalgl pgvgevlarnlaarfgnmdrlleasleelleveevgeltarailetlkdpafrdlvrrlkeagvemeakekggealkgltfvitgelsrpreevkallrrl gakvtdsysrktsylvvgenpgsklekaralgyptlteeelyrlleartgkkaeelv.


4. A method of ligating blunt-end DNA at elevated temperature, comprising using a thermostable blunt-end DNA ligase of claims 1 in a blunt-end DNA ligation reaction at a temperature of about 60° C. or higher.
 5. An article of manufacture, comprising a thermostable blunt-end DNA ligase of claim 1 and a container.
 6. The article of manufacture of claim 5, further comprising a buffer.
 7. A device for analyzing a sample containing DNA, comprising a thermostable blunt-end DNA ligase of claim
 1. 8. The thermostable blunt-end DNA ligase of claim 1, wherein said thermostable blunt-end DNA ligase is suitable for use in a blunt-ended DNA ligation reaction performed at about 75° C.
 9. The thermostable blunt-end DNA ligase of claim 1, wherein said thermostable blunt-end DNA ligase is capable of making concatamers upon multiple ligation events in a blunt-ended DNA ligation reaction performed at about 60° C. or higher.
 10. The thermostable blunt-end DNA ligase of claim 1, wherein said thermostable blunt-end DNA ligase is suitable for use in a nucleic acid amplification scheme which operates at a uniform temperature of about 60° C. or higher.
 11. The thermostable blunt-end DNA ligase of claim 1, wherein said thermostable blunt-end DNA ligase is suitable for use in a nucleic acid amplification scheme which comprises temperature cycling.
 12. The thermostable blunt-end DNA ligase of claim 11, wherein said nucleic acid amplification scheme comprises temperature cycling to temperatures of about 60° C. or higher.
 13. The thermostable blunt-end DNA ligase of claim 11, wherein said nucleic acid amplification scheme comprises temperature cycling from about 94° C. to about 60° C.
 14. The thermostable blunt-end DNA ligase of claim 13, wherein said nucleic acid amplification scheme comprises two or more cycles of temperature cycling.
 15. The thermostable blunt-end DNA ligase of claim 13, wherein said nucleic acid amplification scheme comprises three or more cycles of temperature cycling.
 16. The thermostable blunt-end DNA ligase of claim 1, wherein said fusion protein comprises a component selected from a peptide linker, an N-terminal addition, a C-terminal addition, a tag peptide, a D-amino acid, and a peptide mimetic.
 17. The thermostable blunt-end DNA ligase of claim 1, wherein said fusion protein comprises a component selected from a sugar and a polymer.
 18. The method of claim 4, comprising performing a blunt-end DNA ligation reaction at a temperature of about 60° C. or higher using a thermostable blunt-end DNA ligase selected from the thermostable blunt-end DNA ligase of claim 2 and the thermostable blunt-end DNA ligase of claim
 3. 19. The article of manufacture of claim 5, wherein said thermostable blunt-end DNA ligase is selected from the thermostable blunt-end DNA ligase of claim 2 and the thermostable blunt-end DNA ligase of claim
 3. 20. The device of claim 7, wherein said thermostable blunt-end DNA ligase is selected from the thermostable blunt-end DNA ligase of claim 2 and the thermostable blunt-end DNA ligase of claim
 3. 